244 Part III / Synaptic Transmission
potential. At electrical synapses, the synaptic delay—
the time between the presynaptic spike and the post-
synaptic potential—is remarkably short (Figure 11–2B).
Such a short latency is not possible with chemi-
cal transmission, which requires several biochemical
steps: release of a transmitter from the presynaptic
neuron, diffusion of transmitter molecules across the
synaptic cleft to the postsynaptic cell, binding of trans-
mitter to a specific receptor, and subsequent gating of
ion channels (all described in this and the next chapter).
Only current passing directly from one cell to another
can produce the near-instantaneous transmission
observed at the giant motor electrical synapse.
Another feature of electrical transmission is that
the change in potential of the postsynaptic cell is
directly related to the size and shape of the change in
potential of the presynaptic cell. Even when a weak
subthreshold depolarizing current is injected into the
presynaptic neuron, some current enters the postsyn-
aptic cell and depolarizes it (Figure 11–3). In contrast,
at a chemical synapse, the current in the presynaptic
cell must reach the threshold for an action potential
before it can release transmitter and elicit a response in
the postsynaptic cell.
Most electrical synapses can transmit both depolar-
izing and hyperpolarizing currents. A presynaptic action
potential with a large hyperpolarizing afterpotential
produces a biphasic (depolarizing-hyperpolarizing)
change in potential in the postsynaptic cell. Signal trans-
mission at electrical synapses is similar to the passive
propagation of subthreshold electrical signals along
axons (Chapter 9) and therefore is also referred to as
electrotonic transmission. At some specialized gap junc-
tions, the channels have voltage-dependent gates that
permit them to conduct depolarizing current in only
Figure 11–3 Electrical transmission is graded. It occurs
even when the current in the presynaptic cell is below the
threshold for an action potential.As demonstrated by single-
cell recordings, a subthreshold depolarizing stimulus causes a
passive depolarization in the presynaptic and postsynaptic cells.
(Depolarizing or outward current is indicated by an upward
deflection.)
突触前细胞的
电流脉冲
突触前细胞中
记录的电压
突触后细胞中
记录的电压
one direction, from the presynaptic cell to the postsyn-
aptic cell. These junctions are called rectifying synapses.
(The crayfish giant motor synapse is an example.)
Cells at an Electrical Synapse Are Connected by
Gap-Junction Channels
At an electrical synapse, the pre- and postsynaptic
components are apposed at the gap junction, where
the separation between the two neurons (4 nm) is
much less than the normal nonsynaptic space between
neurons (20 nm). This narrow gap is bridged by
gap-junction channels, specialized protein structures
that conduct ionic current directly from the presynap-
tic to the postsynaptic cell.
A gap-junction channel consists of a pair of
hemichannels, or connexons, one in the presynaptic and
the other in the postsynaptic cell membrane. These
hemichannels thus form a continuous bridge between
the two cells (Figure 11–4). The pore of the channel
has a large diameter of approximately 1.5 nm, much
larger than the 0.3- to 0.5-nm diameter of ion-selective
ligand-gated or voltage-gated channels. The large pore
of gap-junction channels does not discriminate among
inorganic ions and is even wide enough to permit
small organic molecules and experimental markers
such as fluorescent dyes to pass between the two cells.
Each connexon is composed of six identical subu-
nits, called connexins. Connexins in different tissues are
encoded by a large family of 21 separate but related
genes. In mammals, the most common connexon in
neurons is formed from the product of connexin 36.
Connexin genes are named for their predicted molec-
ular weight, in kilodaltons, based on their primary
amino acid sequence. All connexin subunits have an
intracellular N- and C-terminus with four interposed
α-helixes that span the cell membrane (Figure 11–4C).
Many gap-junction channels in different cell
types are formed by the products of different con-
nexin genes and thus respond differently to modu-
latory factors that control their opening and closing.
For example, although most gap-junction channels
close in response to lowered cytoplasmic pH or ele-
vated cytoplasmic Ca
2+
, the sensitivity of different
channel isoforms to these factors varies widely. The
closing of gap-junction channels in response to pH
and Ca
2+
plays an important role in the decoupling
of damaged cells from healthy cells, as damaged cells
contain elevated Ca
2+
levels and a high concentration
of protons. Finally, neurotransmitters released from
nearby chemical synapses can modulate the opening
of gap-junction channels through intracellular meta-
bolic reactions (Chapter 14).
Kandel-Ch11_0237-0253.indd 244 23/12/20 9:51 AM